The present invention relates to a non-aqueous electrolyte secondary battery, and more specifically, to a non-aqueous electrolyte secondary battery using a specific composite oxide of lithium as a positive electrode active material, and a method for charging the battery.
Lithium secondary batteries typical of non-aqueous electrolyte secondary batteries have high electromotive force and high energy density. Because of these features, lithium secondary batteries are now in increasing demand as the main power supply of mobile communication devices and portable electronic devices. Most lithium secondary batteries commercially available at present use as a positive electrode active material LixCoO2 (x represents the lithium content in the active material and changes depending on the charging and discharging of the battery). However, since a cobalt compound used as a raw material is expensive, research and development for other positive electrode active materials is going on to find an alternative to LixCoO2 with a view toward cost reduction. Of these active materials, a composite oxide of lithium LixCoO2 containing nickel as the main component has been studied eagerly and some are already on the market.
Besides cost reduction, lithium secondary batteries are expected to have higher capacity, higher reliability and longer life. To enhance these characteristics, it is possible to improve positive electrode active materials or to set a higher upper limit voltage for charging.
For example, Japanese Unexamined Patent Publication No. 7-320785 discloses a non-aqueous electrolyte secondary battery with a negative electrode containing a carbon material capable of doping and dedoping lithium, a positive electrode containing a composite oxide comprising lithium and a transition metal, and a non-aqueous electrolyte. And, it is proposed to use a graphite material or a carbon material mixture containing graphite as the carbon material, and to use as the above-mentioned composite oxide LixNiyCo1-yO2 (wherein x and y satisfy 0.05≦x≦1.10 and 0.3≦y≦1.0, respectively), and to set an upper limit voltage for charging to 4.1 V or higher. The object of this invention is to provide a non-aqueous electrolyte secondary battery with high energy density and excellent cycle characteristics.
On the other hand, Japanese Unexamined Patent Publication No. 7-335262 discloses a non-aqueous electrolyte secondary battery with a negative electrode containing as a negative electrode active material a carbon material capable of doping and dedoping lithium, a positive electrode containing as a positive electrode active material a composite oxide of lithium and a transition metal, and a non-aqueous electrolyte. And, it is proposed to use a hard carbon material as the negative electrode active material, and to use as the positive electrode active material LixNiyCo1-yO2 (wherein x and y satisfy 0.05≦x≦1.10 and 0.5≦y≦0.95, respectively), and to set an upper limit voltage for charging the non-aqueous electrolyte secondary battery to 4.0 V or higher. The object of this invention is to provide a non-aqueous electrolyte secondary battery with high capacity and excellent storage characteristics.
Furthermore, Japanese Unexamined Patent Publication No. 2001-68168 discloses two conditions about a lithium secondary battery which comprises a negative electrode, a positive electrode, and a lithium salt-containing non-aqueous electrolyte, and which can be charged and discharged for a plural number of times in a reversible manner. As one of the conditions, it is proposed to set the charge end voltage at 4.5 to 4.7 V; to set the average discharge voltage of the positive electrode relative to an Li reference potential at 4.0 to 4.4 V; to set the discharge end voltage of the positive electrode relative to the Li reference potential at 3.2 V or higher; and to set the volume energy density at larger than 430 Wh/l.
As the other condition, it is proposed to set the charge end voltage of the positive electrode relative to the Li reference potential at 4.5 to 4.7 V, and to use a positive electrode active material containing a composite oxide of lithium which satisfies a general formula LiwMvCoxQ1-v-xO2 (wherein M is at least one kind selected from G, Ti, Zr, Y, and Si; Q is at least one kind selected from Ni and Fe; and w, v, and x satisfy 0≦w≦1.2, 0.01≦v≦0.2, and 0.7≦x≦0.95, respectively). The object of this invention is to provide a lithium secondary battery durable to high voltage.
By the way, with the rising of the upper limit voltage for charging, more lithium is deintercalated from a composite oxide of lithium as a positive electrode active material, thereby naturally realizing higher capacity.
On the other hand, however, there are problems of deteriorating the reliability of the battery and decreasing the charge-discharge cycle life of the battery. One cause of deterioration in the battery characteristics is considered that the rising of upper limit voltage for charging causes distortion or fatigue in the composite oxide of lithium from which lithium is deintercalated. Composite oxide of lithium such as LixCoO2 or LixNiO2 reduces in volume crystallographically in the process of a charging reaction during which lithium is deintercalated. In contrast, it increases in volume crystallographically in the process of a discharging reaction during which lithium is intercalated. The rising of the upper limit voltage for charging deintercalates more lithium, thereby causing further reduction in volume. Consequently, repeating charge and discharge at a higher upper limit voltage for charging causes the material to have more distortion and fatigue, thereby remarkably decreasing the charge-discharge cycle life.
As another cause, it is considered that the chemical activation of the material increases. Composite oxide of lithium such as LixCoO2 or LixNiO2, from which a lot of lithium is deintercalated by the rising of upper limit voltage for charging, are put in a highly oxidized state and become excellent in reaction activity. The composite oxide of lithium in this state undesirably reacts with the non-aqueous electrolyte in the battery. Particularly, when the battery is stored at temperatures higher than room temperature, the reaction between the positive electrode active material and the non-aqueous electrolyte deteriorates the battery capacity. In addition, the gas generated by this reaction fills the battery, increasing the internal pressure of the battery and facilitating the deformation of the battery. It is also probable that a safety mechanism provided to prevent battery breakage operates, making it impossible to use the battery. Therefore, in the case where the upper limit voltage for charging is raised to obtain higher capacity, the battery cannot be in the actual use commercially without solving the aforementioned problems.
In view of these aspects, the means proposed in Japanese Unexamined Patent Publication No. 7-320785 cannot offer sufficient reliability of a battery. The inventers of the present invention have found out that the capacity decreases particularly when the battery is stored in a charged condition, and that the recovery rate of the capacity remains low even after several times of charge and discharge. The inventers of the present invention have also found out that the means proposed in Japanese Unexamined Patent Publication No. 7-335262 deteriorates the charge-discharge cycle life. The positive electrode active materials used in these suggestions are composite oxides of lithium, nickel, and cobalt having different composition ratios. When a carbon material mixture containing a graphite material or graphite is used as the negative electrode active material together with either one of the positive electrode active materials, the charge-discharge cycle life is improved, but it is impossible to obtain sufficient storage recovery characteristics. On the other hand, when a hard carbon material is used as the negative electrode active material together with either one of the positive electrode active materials, the storage recovery characteristics are improved, but the charge-discharge cycle life is insufficient. Therefore, it is hard to obtain high reliability and long life at the same time with these positive electrode active materials.
The means proposed in Japanese Unexamined Patent Publication No. 2001-68168 cannot offer sufficient reliability of the battery, either. The inventers of the present invention have found out that the capacity remarkably decreases when it is stored in a charged state. It is also difficult to obtain high reliability and long life at the same time by using the positive electrode active material LiwMvCoxQ1-v-xO2 (wherein M is at least one kind selected from Ge, Ti, Zr, Y, and Si; Q is at least one kind selected from Ni and Fe; and w, v, and x satisfy 0≦w≦1.2, 0.01≦y≦0.2, and 0.7≦x≦0.95, respectively).
Hence, the present invention has an object to provide a non-aqueous electrolyte secondary with high capacity, high reliability, and long life by properly setting the composition of a composite oxide of lithium which is a positive electrode active material and the charging conditions of the battery using this composite oxide of lithium as a positive electrode active material. The present invention has another object to provide a charging method capable of obtaining a non-aqueous electrolyte secondary battery with high reliability and long life in addition to high capacity, when a specific composite oxide of lithium is used as a positive electrode active material.